Depends on how much flavour you want from it I think.
Full flavoured, as in the best Italian oils, would be Waitrose 1 PDO Chianti Classico oil. Pricey, but really special.
Tesco finest Sicilian is close and about half the price of the above. Best value 'special' oil by a way we thought.
Some way below these in flavour, but a good all round dressing oil is Sainsbury's Toscano PGI.
This info is all from Which mag, some time back. We tried all the above, and now buy the Sainsburys when we are near one. It's much lighter in flavour that than two 'winners' but a faultless 'basic' dressing oil, well balanced IMO.